diff --git a/optuna_integration/pytorch_distributed/pytorch_distributed.py b/optuna_integration/pytorch_distributed/pytorch_distributed.py
index e1f3e136..bb9b4cb1 100644
--- a/optuna_integration/pytorch_distributed/pytorch_distributed.py
+++ b/optuna_integration/pytorch_distributed/pytorch_distributed.py
@@ -5,6 +5,7 @@
 from datetime import datetime
 import functools
 from typing import Any
+from typing import cast
 from typing import overload
 from typing import TYPE_CHECKING
 from typing import TypeVar
@@ -125,7 +126,7 @@ def __init__(
                     raise RuntimeError("torch distributed is not initialized.")
                 default_pg: "ProcessGroup" = dist.group.WORLD
                 if dist.get_backend(default_pg) == "nccl":
-                    new_group: "ProcessGroup" = dist.new_group(backend="gloo")
+                    new_group = cast("ProcessGroup", dist.new_group(backend="gloo"))
                     _g_pg = new_group
                 else:
                     _g_pg = default_pg